The present invention relates to improvements in an automobile speed control method and system. More particularly, the invention relates to a speed control method and system for maintaining the speed of an automobile at a constant preselected value.
Speed controls for automobiles are now in commercial use, in which the road speed of the automobile is compared with a preselected value to generate a throttle position feedback signal for controlling the throttle valve at a position at which the road speed balances with the preselected value. Generally speaking, automatic speed control for automobiles requires stabilization of the speed control system. If the stabilization should be insufficient, the speed will fluctuate around the preselected value and this hunting will detrimentally affect the performance of the automobile. Hunting can be prevented by degrading the responsiveness and accuracy of the speed control system. However, hunting can also be prevented without degradation by employing negative feedback control of the throttle valve, as shown in U.S. Pat. No. 3,381,771(Patented May 7, 1968; C1.180-105). In the invention of the aforesaid patent, an actual speed indication signal from the tachometer generator of an automobile speedmeter and a preselected speed command signal from a speed set potentiometer are supplied to a comparator circuit to generate an error signal to control the throttle valve position, and then the error signal is supplied to a differential amplifier circuit together with a throttle position feedback signal(the negative feedback signal) from a feedback potentiometer connected to the throttle valve. An output differential signal from the differential amplifier regulates the position of the throttle valve. The negative feedback stabilizes the movement of the throttle valve and thereby smooths the ride of the automobile. However, there are difficulties in this system for supplying negative feedback of the throttle valve position. It requires a transducer, such as a potentiometer which is linked to the throttle drive system (from the accelerator pedal to the throttle valve) and which generates the throttle position feedback signal, and space to install the transducer is limited (since the throttle drive system is installed in a comparatively narrow space and transducer vibration should be prevented). Also connecting the transducer to the throttle drive system increases the load on the throttle drive system, which then requires more force for operation.